A Multi-Order Power Amplifier (MOPA) is a power amplifier having multiple inputs where an input signal to be amplified by the MOPA is split across the multiple inputs such that the resulting split input signals drive multiple amplification blocks. The split input signals are typically created by analog circuitry or an intelligent digital splitting algorithm. The multiple amplification blocks operate together to produce, at an output of the MOPA, an amplified version of the input signal. Some examples of a MOPA include a 2-way Doherty amplifier, a 3-way Doherty amplifier, a Linear Amplification with Nonlinear Components (LINC) amplifier, an Envelope Elimination and Restoration (EER) amplifier, and a Chireix amplifier.
MOPA operation requires that the split input signals be statically offset in phase, gain, and delay with respect to one another. There are several analog approaches to achieving this control. One approach is Radio Frequency (RF) analog splitting of the input signal as part of an input matching network of the MOPA (e.g., analog Doherty). Another approach is baseband signal splitting followed by analog upconversion, where individual compensation of each upconverter is required to match the gain, phase, and delay in the upconversion path and additional correction circuits and/or algorithms are required to correct for the amplitude and phase imbalance created by analog quadrature modulators.
Splitting the input signal in the analog domain is perhaps the simplest method, but the resulting split is frequency dependent and very limited in capability. Although phase, gain, and delay match across amplification paths can be achieved by physical symmetry of the splitting structure, compensating for any component variations becomes very difficult. This type of split limits the efficiency that can be achieved by a MOPA that has multiple simultaneous inputs and requires independent signal control over a range of frequencies.
Baseband signal splitting has advantages over RF analog splitting but requires that the upconversion chains be matched across the multiple instances for the multiple split input signals so that the split made at baseband remains intact after upconversion. Since upconversion is typically in the analog domain, this is a relatively difficult task for second order MOPAs but is extremely difficult for higher order MOPAs. Further, compensating gain, phase, and delay is frequency dependent and, even worse, is physical realization dependent (e.g., every unit built needs to be calibrated differently or an average calibration is used for all units which limits achievable performance).
One issue with existing approaches to offsetting the gain, phase, and delay of the split input signals provided to a MOPA is accuracy and complexity. As the order (i.e., the number of inputs) of the MOPA increases, the complexity of existing solutions becomes nearly insurmountable. Another issue with existing approaches is that they are frequency dependent. As such, they are not suitable for multi-band input signals.